1. Field of the Invention
This invention relates in general to a head positioning control systems in magnetic storage systems, and more particularly to a method and apparatus for servowriting using a unipolar write current.
2. Description of Related Art
Computer manufacturers have always worked to squeeze more data into smaller spaces. That mission has produced competing and complementary data storage technology including electronic circuits, magnetic media like hard disks and tape, and optical media such as compact disks. Today, companies constantly push the limits of these technologies to improve their speed, reliability, and throughput all while reducing cost. Storage technology has come a long way, and manufacturers continue to improve its speed, reliability, and throughput. Hard disks are the most common type of mass storage device today thanks to their low cost, high speed, and relatively high storage capacity.
One important improvement in hard drives related to increased data density has been in the servo systems. In hard drives, bytes of data are stored on the magnetic medium in sectors. Reading or writing a sector requires knowing the physical location of the data on the media so that the servo controllers can position the read/write heads in the correct location at the appropriate time. Improvements in servo positioning have directly contributed to higher track densities, while advances in rotational position measurement and speed control have sustained track utilization efficiency.
In early disk drives, radial position of data was determined mechanically. Sometimes, open loop stepping systems were used to move the heads, while other drives used relatively crude analog servo systems with feedback from optical or electromagnetic sensors. In such techniques, the number of tracks in a drive was limited by the mechanical resolution, accuracy, and repeatability of the electrical and mechanical components.
Dedicated servo technology began replacing mechanical head positioning starting in the late 1960s. This led to a major increase in attainable track densities. Drives with dedicated servo systems used an entire recording surface to store a set of very accurate servo tracks. As the platters rotated, a head dedicated to the servo surface read the reference track, which determined and controlled the radial position of the head. Since the servo-tracking head was attached to the same actuator mechanism as the read/write heads for data on the other disk surfaces, the exact radial position of all vertically stacked heads was guaranteed. Thus, dedicated servo surfaces allowed the data tracks to be placed more closely together, thereby allowing significant increases in overall data density.
While the dedicated-servo technique significantly increased track density, it did so at the cost of an entire surface of storage space. As track densities increased, other drive design challenges surfaced, e.g., variations in temperature from one platter to another caused a physical difference in the location of tracks from platter to platter. Thus, it became difficult or even impossible to locate tracks of previously recorded data if the ambient temperature changed. So, in the late 1970s, drive designers began investigating and employing several techniques for embedding servo information on each disk surface.
With embedded servo technology, fixed information identifying the radial-head position is pre-recorded in specific areas on all platter surfaces during the manufacturing process. In early disk drives, a scheme that combined both a dedicated servo surface and some embedded servo information was used to form a so-called hybrid servo system. Eventually, the dedicated servo surface was eliminated.
As with dedicated servos, the embedded servo information took up significant recording space within the overall drive volume. But it allowed the drive to continually recalibrate itself, thereby permitting much higher recorded track densities, even in varying ambient temperatures. This gain in total capacity exceeded the capacity overhead required for the servo-control information.
Now, embedded servo information is pre-recorded in dedicated radial spokes, thus dividing the available data-recording space on a platter's surface into fixed wedges. Controlling the number of wedges allows a designer to optimize the media design by trading off head-positioning bandwidth for available storage capacity.
The techniques for determining the rotational position of recorded data has also improved. For example, the mechanical sensing techniques of the earliest drives determined the angular displacement of a data field from an arbitrary fixed point on a disk surface in addition to sensing radial head position. A series of symbol on the circumference of one platter were used for the measurement, with mechanical tolerances again limiting the resolution. Each symbol corresponded to a recorded data sector. When improvements to heads and media subsequently allowed increases per track data density, the fixed mechanical symbols could not be easily changed to allow the disk drive to harness the increased data density.
In the mid-1960s, designers developed recorded headers, which replaced mechanical systems. In doing so, areal densities increased significantly. As noted earlier, headers are simply small data blocks placed in front of each data sector during formatting.
The header area contained the track number, the head number, and the number of the sector that immediately followed the header. Header information allowed the disk controller to confirm the head and the radial track. It also allowed accurate determination of the sector location where the data should be recorded or read. In disk drives with embedded servo fields, the header was expanded to include information that allowed a sector to be split. Accordingly, a part of the sector could be before a servo field and the remaining portion after the servo field. In some systems, a sector was split several times because of sector-size or data-field limitations.
When a read or write is performed, the headers are read and compared with the calculated header to ensure that the correct data field has been located. If the track-number field is incorrect, the servo system is alerted so that the head can be repositioned to the correct track. Similarly, if the head field is incorrect, the drive electronics can select the correct head. If the sector number is incorrect, the controller must merely wait while the disk rotates, until the correct header moves under the read/write head.
Because the track number is confirmed before reading or writing occurs, track density can be improved even beyond the point where the servo system can guarantee exact positioning because positioning errors can be detected and recovered without impacting data reliability. Thus, the headers improve overall reliability by preventing reading or writing of data at wrong locations on the disks.
Prior to the advent of headers, sectors had to be sufficiently distanced from one another to allow for mechanical tolerances and variability. Hence, there were large gaps between sectors; headers narrowed these gaps. Compared with older drives, the storage area used for the headers was almost free. But the improvements in overall data density, due to increased bit and track densities, were significant.
Recent advances in head design, specifically the development of MR heads, significantly increased the header overhead because of their physical design, i.e., MR heads require double headers to accommodate the physical separation of read (MR) and write (inductive) elements of the heads. Accordingly, with MR heads, as much as ten percent of a drive's total storage capacity can be dedicated to the header information.
Today, the header is often eliminated entirely. This allows the drive controller to make full use of information stored in the embedded servo field. With that information, the controller can determine the correct combination of head, cylinder, and sector that uniquely designates the location of a data field recorded on the media. The controller can find the head position and the sector location within each servo-defined wedge. Use of the servo synchronization field to recalibrate internal logic ensures accuracy.
The sector layout of the headerless-formatting system looks much like that of header-based systems, but with the obvious lack of a header. Headerless formatting technique uses new information (stored in the embedded servo field) to exactly determine the rotational position of any and all sectors. The servo field contains a servo synchronization pulse used to determine the current wedge position. The sector location within each wedge, relative to the nearest servo sync pulses, is governed by rotational speed. That allows the controller to set a fixed window, with a width that depends on the speed tolerance of the drive, to test the validity of sync pulses received. Multiple track formats are supported, allowing the controller to support multiple zones. That, in turn, permits maximum recording densities to be used throughout the media, while maintaining the integrity of data recovery.
For maximum bit packing, the headerless formatting system allows sectors to be split anywhere after the first byte of data or before the last byte of data in the ECC fields. Thus, the headerless scheme represents a tremendous improvement over header-based systems, which limited sector splitting because of difficulty in handling sector fragments.
The embedded servo field typically includes a digital field containing cylinder number information (indicating which track a head is positioned over) with additional bits indicating the current head being read and the current servo-field number.
Writing servo patterns on a DC erased disk produces servo patterns made of two alternating transition types. One is produced along the trailing edge of the write-gap. The other transition is produced along the leading edge of the write-gap.
However, the side erase band and the curved transition at the track edges have a detrimental effect on the servo signal. These edge effects are usually controlled by reducing the write current during servowrite and also by careful head design and using high coercivity disks. Still, the side effects are not eliminated.
It can be seen that there is a need for a method and apparatus that produces servo patterns while reducing the edge effects.
It can also be seen that there is a need for a method and apparatus that increases the written track width beyond that produced by the conventional bipolar write current during servowriting.